Version dependency

Seen on a recent install:

```
     NB: this package now depends on R (>= 4.1.0)
     WARNING: Added dependency on R >= 4.1.0 because package code uses the
     pipe |> or function shorthand \(...) syntax added in R 4.1.0.
     File(s) using such syntax:
       ‘clean_f_name.Rd’ ‘step_climate.R’ ‘step_epi_slide.Rd’
```